An igneous fragment from cluster IDP L2011#21: An analog for the source of pyrrhotite and taenite in comet 81P/Wild 2 captured in Stardust aerogel


  • Frans J. M. Rietmeijer

    Corresponding author
    • Department of Earth and Planetary Sciences, MSC03-2040, 1-University of New Mexico, Albuquerque, New Mexico, USA
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The silica glass extracted from the bulbous parts of Stardust tracks is riddled by electron-opaque nanograins with compositions that are mostly between pyrrhotite and metallic iron with many fewer nanograins having a Fe-Ni-S composition. Pure taenite nanograins are extremely rare, but exist among the terminal particles. Assuming that these Fe-Ni-S compositions are due to mixing of pyrrhotite and taenite melt droplets, it is remarkable that the taenite melt grains had discrete Fe/Ni ratios. This paper presents the data from an igneous pyrrhotite/taenite fragment of cluster IDP L2011#21, wherein the taenite compositions have the same discrete Fe/Ni clusters as those inferred for the Stardust nanograins. These Fe/Ni clusters are a subsolidus feature with compositions that are constrained by the Fe-Ni phase diagram. They formed during cooling of the parent body of this cluster IDP fragment. These specific Fe/Ni ratios, 12.5, 24, 40, and 53 atom% Ni, were preserved in asteroidal taenite that survived radially outward transport to the Kuiper Belt where it accreted into the (future) comet Wild 2 nucleus.